Numerical Simulation And Experimental Research On Turning Process For Laser-assisted Machining Of Fused Silica

Fused silica is an amorphous material composed of extremely high purity SiO₂.Its long-range disordered silicon-oxygen atomic structure gives it excellent thermal stability,corrosion resistance,mechanical properties and spectral characteristics,and is widely used in various optical instruments,precision equipment and various special working environments.However,for fused silica with high hardness and brittleness and low fracture toughness,traditional grinding and polishing processes are difficult to achieve high processing quality efficiently,which limits the development of materials to a certain extent.Laser-assisted machining is a method that can enhance the flow properties of brittle materials through heat-assisted methods to reduce the processing difficulty,which provides a new idea for improving the processability of fused silica.In this paper,based on the mechanism of the LAM method in the turning of fused silica,the temperature field,turning simulation and process experiments are studied.The main research contents are as follows:Firstly,based on the CO₂ laser’s effect on fused silica,the theory of heat conduction and the thermal properties of fused silica materials,a cylindrical three-dimensional transient heat transfer model was established in COMSOL,and the influence of processing parameters and laser parameters on the surface temperature of the workpiece was analyzed.And the verification of temperature test was carried out respectively,the results showed that the temperature measurement test and the simulation result were in good agreement.To simulate the temperature law of the material touched by the tool during the actual turning process,the workpiece geometry model based on different working conditions was established,and the orthogonal simulation based on the five factors of power,feed,depth of cut,speed,and tool-laser included angle was carried out to study the influence of various parameters on the temperature in the shear zone.Finally,a mathematical model of the temperature in the shear zone was fitted based on theπtheorem,and suitable processing parameters were initially optimized.Secondly,the constitutive model of fused silica based on Oxley’s cutting theory is used to establish the field distribution model of unequal shear zone stress,strain,strain rate,and temperature.Orthogonal cutting experiments under different powers were carried out,combined with the results of SHPB experiments,the optimal Johnson-Cook constitutive parameters were obtained through genetic algorithm.A cylindrical sequential thermo-mechanical coupling LAM turning model based on the constitutive parameters was established in ABAQUS,and the dynamic process of fused silica in LAT was analyzed.Combined with turning experiments,it was proved that the high-temperature constitutive model of reverse identification has good accuracy.The influence of different process parameters on chip morphology and cutting force and the mechanism of material removal are studied.The simulation results show that the high temperature generated by thermal assistance enhances the viscoplasticity of the material,significantly reduces the cutting force,and the chip morphology is semi-continuous and continuous.It is believed that the material removal mechanism under LAM conditions is a brittle-plastic mixed mode.Finally,an annular tool holder with adjustable laser beam-tool circumferential angle is designed,and the influence of laser power and circumferential included angle on the quality of the machined surface is studied and analyzed.The experimental results show that higher power can significantly improve the machinability of the material,and the quality of the processed surface is significantly improved.However,too small a circumferential included angle will cause the chips to adhere to the machined surface,resulting in instability of the machining process and poor surface quality.Furthermore,the optimal combination of process parameters was obtained through Taguchi orthogonal experiment.The verification experiment showed that the turning experiment corresponding to the optimal parameter combination obtained better surface quality and lower cutting force.